docs: def usable_ids(cls, id, accept_multi=True):
          """ Retrieve id from input which can be an id or a cn."""
          try:
              qry_id = [int(id)]
          except ValueError:
              try:
                  qry_id = cls.from_cn(id)
              except Exception:
                  qry_id = None
  
          if not qry_id or not accept_multi and len(qry_id) != 1:
              msg = 'unknown identifier %s' % id
              cls.error(msg)
  
          return qry_id if accept_multi else qry_id[0]